SEA 222 Operator's Manual page 52

5.5
5.5.1
THE PHASE LOCKED LOCAL OSCILLATOR SYSTEM
BLOCK OIAGRA.M
Figure 5.5.1 shows the block diagram of tpe phase locked local oscil-
lator system of the SEA 222.
A two loop system is used in the SEA 222 consisting of the high
frequency loop, operating with a 6.4 KHz reference frequency and the
low frequency loop which operates with a 100 Hz reference. The com-
bination of two loops provides 100 Hz resolution over the high fre-
quency spectrum, along with reasonable loop switching and settling
times.
The high frequency loop starts with the VHF VCO (Voltage Controlled
Oscillator). This oscillator actually consists of two separate oscil-
lators, where the tuning range is divided into two approximately equal
segments. The use of two osci
11
ators, bandswitched by the control
computer, provides good VCO spectral purity over the desired tuning
range of 47 - 68 MHz.
~oth
VCOs are varactor tuned by an output
voltage from the phase detector in synthesizer chip, Ull.
The signal from the selected VCO is buffered and then applied to the
first signal mixer, Al. Simultaneously, a sample of the VCO output is
applied to the high speed loop dual modulus prescaler, U12.
The low speed loop starts with the 38.6 MHz VCXO, 015. This oscil-
lator operates over the range of 38.5984 to 38.6047 MHz and is varactor
·tuned by an output voltage from the phasedetector in synthesizer chip
Ul6.
The VCXO signal is buffered by Ql6 and then applied to the second
signal mixer, A2. Simultaneously, a sample of the VCXO signal is
applied to one of the inputs of an HCMOS exclusive OR gate. A buffered
6.4 MHz signal from the master clock osci 11 ator is applied to the
second input of the exclusive OR gate which acts as a harmonic mixer.
The output signal is the difference between the VCXO signal and the 6th
harmonic of the clock signal at 38.4 MHz, or approximately 200 KHz.
This 200 KHz signal is filtered, buffered and then applied to the
signal input of the low speed loop synthesizer chip, U16.
From the above discussion
it
may be seen that ALL of the interna 1
conversion oscillator frequencies are phase locked to the 6400.0 KHz
master clock oscillator, that the first conversion oscillator consists
of a VCO which operates as a "coarse tuning" oscillator having a basic
resolution of 6.4 KHz and that, finally, the second conversion oscil-
lator operates as a "fine tuning" loop which tunes 6.3 KHz in 100 Hertz
steps.
31)